Today's subscriber line interface circuits should imitate the traditional way of feeding a telephone line from an exchange battery via a feeding resistance. Thus, this feeding resistance determines the dependence of the line current on the line voltage, i.e. the line feed characteristic of the line interface circuit.
However, different countries require different feeding resistances, which results in that today's line interface circuits are not generally usable but have to be adapted to the requirements specified in the respective county
For a short-circuited line and for low-resistance line loads, the line current will be high which causes a high power dissipation in the feeding resistance. To avoid this, it is known to limit the maximum value of the line current. Also, this maximum value differs from country to country and, consequently, today's line interface circuits have to be adapted also in this respect to the requirements specified in the respective country. However, in today's line interface circuits, it is not possible to arbitrarily limit the line current to desired values in a simple way.
When the line is open, i.e. with the handset on-hook, today's line interface circuits have the disadvantage that the line voltage can be lower than the expected, ideal line voltage for a given feeding voltage. This can depend e.g. on the fact that the associated line may have a leakage resistance or that the device, e.g. a telephone set, connected to the line draws current from the line for some reason. The value of this open-line voltage is, however, very important for some units, e.g. so-called MTUs (Maintenance and Test Units) and certain facsimile apparatuses.
Moreover, today's line interface circuits are not adapted to adapt their line feed characteristic to possible feeding voltage or supply voltage variations.